Protection of niobium



United Horizons Incorporated, Cleveland, Ohio, at New Jersey corporation No Drawing. Filed Nov. 6, 1958, Ser. No. 772,180 7 Claims. (Cl. 29-194) This invention relates to the protection of niobium and similar metals from oxidation. More particularly it relates to a process whereby a protective coating is produced on niobium or on a similar substrate and to the articles produced by said process.

Niobium and niobium base alloys possess high temperatures strength properties which make these metals desirable for many end uses. Unfortunately when pure niobium is exposed to oxidation at high temperatures, oxide scales form on the surface ranging in composition from Nb O to and including Nb O With continued exposure, or with intensive oxidation, the scales formed become thick and flake olf, and thereby expose a fresh surface to further oxidation. As a result, the article is ultimately weakened to the point of failure.

Early efforts to overcome this undesirable tendency were directed to alloying the niobium to improve its resistance to oxidation. Unfortunately in many instances alloys which exhibited improved oxidation resistance were found to be extremely difficult to fabricate and as a result, the benefits conferred by alloying were not always realizable. In general it may be stated that the addition of elements known to enhance the resistance of niobium to oxidation, usually results in an alloy which is brittle and which does not fabricate readily.

One previously proposed means for overcoming this poor oxidation resistance has been to clad the niobium 0r niobium base alloy article with a suitable oxidation resistant alloy. This approach has not been entirely successful because diffusion between the cladding material and the niobium base has often produced brittle interfaces which did not Withstand repeated thermal cycling andwhich in many instances ultimately failed by actual physical separation of the clad from the base.

Another objection to roll cladding is that structural members cut from clad material bear unclad edge portions which must be laboriously hand welded to prevent failure of the protective layer by immediate oxidation along such bare surface portions.

Still another disadvantage in the prior efforts to clad niobium and niobium base materials has resided in the brittleness of the cladding materials themselves at the temperatures of fabrication, e.g. at roll cladding temperatures. This has seriously limited the use of such coatings in applications where the article to be protected is subjected to bending stresses.

Other methods of applying protective coatings to niobium and niobium-base alloys are frequently found to be of limited applicability because of the configuration of the article to be protected.

One object of this invention is to provide a means for protecting niobium and niobium base alloys against oxidation at elevated temperatures.

This and other objects are accomplished in accordance with the present invention wherein a matrix of at least one metal such as nickel, cobalt, chromium or iron and a metal powder, oxide or compound are simultaneously deposited onto a niobium surface and the resulting clad article is subjected to heat treatment of the surface. This procedure has been found to produce a metal-metal compound composite coating layer which not only adhered tenaciously to the substrate but which, when suitable materials were selected, protected the niobium against the oxidizing effects of oxidizing atmospheres at temperatures as high as 2500" F.

3,057,048 Patented Oct. 9, 1962 More particularly, it has been found that a suitable coating for the protection of niobium and niobium base alloy substrates could be produced by electrodeposition of a metal such as nickel, cobalt, chromium, or iron as a matrix and simultaneously therewith, the occlusion of various materials within specified relative proportions therein, followed by a suitable after-treatment.

Nickel is preferred as the metal serving as a matrix in the coating deposited on the niobium base material, although it would appear that other oxidation-resistant metals such as chromium, cobalt and the precious metals could be used with similar results. Because it is readily available and because of its suitability, nickel is preferred as the matrix metal in which up to about 20% by volume of various materials are occluded.

By selecting an appropriate combination of matrix metal and occluded material, it is possible to produce coatings possessing very specific properties. For example, it would appear that coatings having a coeflicient of thermal expansion matching that of the base material, or differing from it in a specific manner, may be pro- .duced. Alternatively, coatings which exchange metal and/or oxygen ions with the substrateat controlled rates-may be produced, or coatings which are particularly tenacious, or ductile or which possess any other especially desired property.

The occluded material is preferably one or more refractory metals, refractory metal alloys or refractory metal compounds. Chromium and silicon are examples of metals which have been found suitable. Chromium silicide, ferroboron and nickel boron have been successfully used as refractory metal alloys. Oxides, borides and silicides have been used as the refractory compounds.

While I do not wish to be bound by any specific theory it is believed that many of the above refractory materials are charcterized by a low solubility in the matrix metal and the resulting coating is characterized by both oxidation resistance and improved thermal stability.

A typical processing schedule for the production of oxidation resistant coatings by occlusion of specified solid particles in an electro-deposited nickel film included the following:

(1) Preparation of niobium cathode:

(a) Sanding with No. 3 emery paper, then (b) Scrubbing with abrasive detergent, then (0) Wash in concentrated (or 1:1) HCl to neutralize the specimen surface.

(2) Plating:

(a) Immediately thereafter place cathode in Watts nickel bath operated at from 40 C. to 70 C. and containing selected additives.

(b) Electroplate at 20-100 amperes/sq. ft. of cathode area for from /2 up to 4 hours, rotating cathode slowly (4 to 6 rpm), using bagged nickel anodes. Reverse piece occasionally to insure equal coverage on all surfaces, reversal being effected without interruption of the process.

(0) Agitate bath with argon or compressed air.

(d) Maintain pH at between 2 and 5, depending on composition of suspended material.

(3) Post Treatment:

(a) Water Wash.

(11) Dry.

(0) Heat treatment:

(1) Hot dip in molten A1 at 900 C. to provide an oxidation barrier during annealing in air at 2000 F. to 2500 F. for about 1 hour.

(2) Anneal in inert atmosphere at 2000 F. to

2500 F. for about 1 hour.

The following examples will serve to further illustrate specific embodiments of the practice of my invention:

Example 1 A mixture was prepared of dry, finely divided (minus 325 mesh, Tyler Standard) metallic chromium, chromium silicide, nickel boron and ferroboron, in the relative proportions of about :5:5:3 (by Weight). The mixture was leached with a hot solution of Watts nickel plating bath, to remove any constituents soluble in the plating bath.

A piece of 0.3 inch diameter niobium rod was prepared for plating by first grinding with No. 3 emery paper, then scrubbing with an abrasive cleaner, then rinsing in water and finally dipping in 1:1 HCl. After the cathode was washed in the HCl, it was inserted into a standard Watts nickel bath in which there had been suspended, about 200 grams of the leached mixture per liter of Watts bath. It was thereafter plated therein at a pH of 3 using a nickel anode wrapped in canvas. The cathode was rotated at 4 r.p.m. and the bath was continuously agitated with compressed air for the 44 minutes during which the specimen was plated with a coating about 11 mils thick. The cathode current density was about 100 amps. per square foot. The specimen was thereafter removed, dried and then heat treated in air at about 900 C. to 1000 C. for 5 minutes.

The piece was heated to about 2500 F. and maintained at that temperature in a stream of air flowing through the test chamber at the rate of cubic feet per hour.

This sample showed about a 20 fold increase in oxidation resistance as compared to uncoated niobium in a similar test chamber. Uncoated niobium shows signs of rapid oxidation almost immediately upon exposure to the test conditions.

Other pieces prepared in similar fashion were examined before testing and found to contain between and by volume of the suspended material added to the nickel bath, in a matrix of nickel about 10 mils in thickness. Such pieces survived up to 19 hours exposure at 1100" C. in a 10 cubic foot per hour air stream before oxidation of the substrate niobium commenced.

Example 2 A mixture of chromium and silicon in the relative proportions of 4:1 (by Weight) was prepared by blending finely powdered Cr and Si in a mortar. The mixture was leached with hot plating solution (Watts nickel bath) to minimize contamination, and the leached solid material was then added to an unused portion of Watts-type nickel bath in the proportion of 200 grams of mixture for each liter of electrolyte.

A niobium cathode cleaned as in Example 1 was given a flash coat of nickel in a Watts-type bath at pH of 2 for 1 minute and then plated in a Watts bath of pH 5 containing the suspension of Cr and Si. Plating was carried out at 40 C. at 20 amps. per square foot of cathode area, for 150 minutes, While slowly rotating the cathode. The cathode was reversed and the clamped end was then cleaned and plated in the same manner. A slightly rough 15 mil coating was produced.

The piece was then hot dipped in molten A1 at 900 C. for 5 minutes. When heated in air at 2500 F., it exhibited a weight gain of only 2.6 mg./sq. cm./hr. Uncoated niobium exhibited weight gains of between 28 to 36 mg./ sq. cm./hr. when subjected to the same environment.

Example 3 A mixture of powdered silica, alumina and thoria in the relative proportions of 12:20:1 was prepared by mix ing dry, minus 325 mesh (Tyler Standard), material in a mortar. The mixture was leached with hot Watts nickel bath electrolyte and then added to fresh electrolyte, between about 50 and 100 grams being suspended in one liter of electrolyte.

A niobium rod, cleaned as described above and flashed in a Watts-type nickel bath at pH 2 for 1 minute, was plated in a Watts bath containing the suspended oxides, maintained at 40-45" C. and at a pH of 2 and using amps. per sq. ft., cathode current density. Air agitation was used. The anode was nickel, Wrapped in canvas, and the cathode was rotated to produce an even plate thereon. After 2 hours, a smooth uniform plate was obtained, containing about 20 volume percent of occluded oxide in a nickel matrix.

After drying, the piece was hot dipped in Al at 900 C. The resulting piece survived 10 one-hour cycles in air at 2500 F. and showed no signs of failure.

It will be seen that niobium pieces coated as described above were resistant to oxidation even when subjected to severe conditions.

One possible explanation for the enhanced resistance to oxidation exhibited by niobium, coated with a deposit consisting of a matrix of nickel and between about 5% and 20% by volume of occluded particles of the nature specified, may be that tenacious mullite-type or spineltype scales are formed depending on the nature of the coating material, and the environment to which it has been subjected.

It has been found that the amount of occluded material may be varied between 5% and 20% by volume, without impairment of the protection but I prefer about 15 by volume of occluded material in a nickel matrix.

With most additives, it has been found that an increase in the amount of occluded material may be favored by the following:

( 1) Increased amount of suspended material.

(2) Decreased movement of cathode surface relative to the plating bath.

(3) Decrease in particle sizes of material in suspension.

(4) Increased cathode current density, within the normal plating range.

(5) Low pH within normal plating range.

Having now described my invention in accordance with the patent statutes,

I claim:

1. A method of producing an oxidation resistant coating on a niobium base material which includes preparing an electroplating bath of one of the metals of the group consisting of Fe, Co, Ni and Cr having suspended therein finely divided particles of solid material selected from the group consisting of alumina, silica, thoria, silicon, aluminum, chromium, boron, and combinations thereof; maintaining said solids in suspension in said bath while electrodepositing a matrix consisting of the metal of said electroplating bath on said niobium base material, said matrix containing between about 5% and 20% by volume of occluded particles of said suspended solid material, which is sufficient to enhance its inherent oxidation resistance and therefore its protective capacity for the niobium-base material.

2. The method of claim 1 wherein the occluded solids comprise 15 by volume of said coating.

3. The method of claim 1 wherein the base bearing the coating is subjected to a temperature of between about 2000 F. and 2500 F. in an inert atmosphere for about one hour to enhance its oxidation resistance.

4. The method of claim 1 wherein the niobium-base material is coated with a nickel matrix in which chromium and silicon are occluded, by deposition from a nickel plating bath containing a suspension of finely divided chromium, chromium silicide, nickel boron and ferroboron.

5. The method of claim 1 wherein the coating bath comprises a nickel plating bath containing a suspension of finely divided chromium and finely divided silicon.

6. The method of claim 1 wherein the coating bath comprises a nickel plating bath containing a suspension of silica, alumina and thoria.

7. A niobium base article coated with an adherent continuous electrolytically deposited coating comprising a metallic matrix formed of a metal selected from the group consisting of Fe, Co, Ni and Cr and wherein there is occluded between 5% and 20% by volume of a refractory material selected from the group of materials which are insoluble in the coating bath and consisting of alumina, silica, 'thoria, silicon, aluminum, chromium, boron and combinations thereof.

References Cited in the file of this patent UNITED STATES PATENTS 1,504,206 Baker Aug. 12, 1924 6 Kristensen Oct. 13, 1931 Clark Feb. 8, 1949 Beach June 21, 1955 Went June 19, 1956 Campbell Nov. 27, 195 6 Slatin May 5, 1959 Wehrman Feb. 9, 1960 

1. A METHOD OF PRODUCING AN OXIDATION RESISTANT COATING ON A NIOBIUM BASE MATERIAL WHICH INCLUDES PREPARING AN ELECTROPLATING BATH OF ONE OF THE METALS OF THE GROUP CONSISTING OF FE, CO, NI, AND CR HAVING SUSPENDED THEREIN FINELY DIVIDED PARTICLES OF SOLID MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALUMINA, SILICA, THORIA, SILICONE, ALUMINUM, CHROMIUM, BORON, AND COMBINATIONS THEREOF; MAINTAINING SAID SOLIDS IN SUSPENSION IN SAID BATH WHILE ELECTRODEPLATING A MATRIX CONSISTING OF THE METAL OF SAID ELECTROPLATING BATH ON SAID NIOBIUM BASE MATERIAL, SAID MATRIX CONTAINING BETWEEN ABOUT 5% AND 20% BY VOLUME OF OCCLUDED PARTICLES OF SAID SUSPENDED SOLID MATERIAL, WHICH IS SUFFICIENT TO ENHANCE ITS INHERENT OXIDATION RESISTANCE AND THEREFORE TIS CAPACITY FOR THE NIOBIUM-BASE METERIAL. 